Efficient and Stable Perovskite Solar Cells Based on Inorganic Hole Transport Materials

Although power conversion efficiencies of organic-inorganic lead halide perovskite solar cells (PSCs) are approaching those of single-crystal silicon solar cells, the working device stability due to internal and external factors, such as light, temperature, and moisture, is still a key issue to address. The current world-record efficiency of PSCs is based on organic hole transport materials, which are usually susceptible to degradation from heat and diffusion of dopants. A simple solution would be to replace the generally used organic hole transport layers (HTLs) with a more stable inorganic material. This review article summarizes recent contributions of inorganic hole transport materials to PSC development, focusing on aspects of device performance and long-term stability. Future research directions of inorganic HTLs in the progress of PSC research and challenges still remaining will also be discussed.


Introduction
Photovoltaic (PV) technology research has been mainly focused on high efficiencies with low-cost materials and fabrication processes, but the high stability of working devices is also crucial for commercialization. Perovskite solar cells (PSCs) have impressively increased their unit-cell efficiency from 3.8% to 25.5% within about a decade [1], approaching single-crystal silicon solar cell efficiency values. Organometallic halide perovskites are based on the chemical formula of AMX 3 , where A is organic or metal cations, such as formamidinium ((NH 2 ) 2 CH + (FA + )), methylammonium (CH 3 NH 3 + (MA + )), or Cs + , M is metal ions such as Pb 2+ or Sn 2+ , and X is halogen ions, such as I − , Br − , or Cl − . Organometallic halide perovskites exhibit features of an ideal light absorber material, including high absorption coefficients (~10 −4 cm −1 ), long carrier diffusion lengths (>1 µm), ambipolar charge transport capabilities, and low exciton binding energy (20-50 meV) [2].
Despite the remarkable growth in efficiency enhancement, compared to silicon solar cells, the working device stability still needs improvement. Degradation in PSCs can occur from both internal and external factors. Internal degradation factors include ion migration in the perovskite, lattice relaxation at interfaces, and diffusion of HTL dopants, while external degradation factors include exposure to light, heat, bias, moisture, and oxygen [3].
Perovskite solar cell device structures normally consist of the perovskite layer in between an HTL and electron transport layer (ETL), with a transparent conducting oxide (TCO), such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), on top of a glass substrate and a metal, such as silver (Ag), gold (Au), or aluminum (Al), as the top contact [4]. Depending on the sequence of the transport layer type, the device structure is called n-i-p for the n-type ETL on the bottom (on top of the TCO) and the p-type HTL on top of the light-absorbing perovskite layer (below the top metal contact), and p-i-n for the HTL on the bottom and ETL on top of the light-absorbing layer.
The conventional HTL is based on organic materials, which are susceptible to elevated temperatures, and diffusion of HTL dopants into the perovskite layer can also cause degra-

Nickel Oxide
Nickel oxide has already been used in dye-sensitized solar cells (DSSCs) and organic photovoltaics (OPV) as the p-type HTL before being applied in PSCs [62,63]. NiO x exhibits high transparency from its wide bandgap (3.6 eV), deep valence band (−5.2 to −5.4 eV), and high carrier mobility (0.1 cm 2 /Vs) while having suitable stability against light, heat, and moisture, making it a suitable HTL candidate for PSCs [64]. NiO x has been applied to PSCs in both n-i-p and p-i-n structures. There are more considerations required when applying NiO x on top of the perovskite layer in n-i-p structures, as sputtering, water, or polar organic solvents can damage the underlying perovskite layer. NiO x was applied in n-i-p structured PSCs by dc magnetron sputtering [17]. The efficiency was only 7.3%, but the unencapsulated device remained stable for 60 days in a 25 • C ambient atmosphere with 28 ± 2% relative humidity without light soaking. Chlorobenzene-dispersed NiO x HTL can also be directly deposited on the perovskite films without decomposing the perovskite, resulting in efficiencies over 9% [18].
For p-i-n configurations, power conversion efficiencies (PCEs) of 17.3% [19] and 17.6% [20] were achieved by pulsed laser deposition, by controlling the deposition time and oxygen partial pressure, and by solution processing, respectively. Vertical recrystallization of the perovskite layer and co-doping the NiO x with lithium (Li) and magnesium (Mg) resulted in a PCE over 20%, and the device stability maintained over 85% of its initial PCE under maximum power point tracking (MPPT) conditions for over 500 h, as shown in Figure 1 [21]. Li-doping increases the p-type conductivity, whereas Mg-doping adjusts the valence band energy level. Previous reports demonstrate NiO x -based PSCs maintaining 85% of its initial PCE at 60% humidity and 60 • C for 500 h for encapsulated devices ( Figure 2) [60], and 100% of its initial PCE under 1 SUN at 40% humidity and 35 • C for 1000 h under MPPT conditions for devices without encapsulation ( Figure 3) [61].
Application of NiO x by atmospheric pressure spatial atomic layer deposition (s-ALD) system, a more rapid method than conventional ALD [65,66], in p-i-n structured PSCs has been demonstrated. Employing high-quality and high-uniformity NiO x HTLs in PSCs resulted in PCEs over 17% with negligible hysteresis and fill factors over 80% [22,67]. Perovskite films with improved efficient collection of charge carriers and intrinsic electronic quality were enabled from the high uniformity of NiO x , resulting in PSC devices with reduced interfacial trapping and improved open-circuit voltage (V OC ). NiO has also been applied to p-i-n PSCs using plasma-enhanced ALD (PEALD), resulting in PSCs over 17% [24]. For p-i-n structured PSCs, atomic layer deposition (ALD) of NiO employed in PSCs resulted in efficiencies of over 18% [23]. The ALD NiO-based PSCs maintained over 99% of their initial PCE at room temperature conditions and 87% at 85 • C under 1 SUN, MPPT, as shown in Figure 4. 85% of its initial PCE under maximum power point tracking (MPPT) conditions for over 500 h, as shown in Figure 1 [21]. Li-doping increases the p-type conductivity, whereas Mgdoping adjusts the valence band energy level. Previous reports demonstrate NiOx-based PSCs maintaining 85% of its initial PCE at 60% humidity and 60 °C for 500 h for encapsulated devices (Figure 2) [60], and 100% of its initial PCE under 1 SUN at 40% humidity and 35 °C for 1000 h under MPPT conditions for devices without encapsulation  Application of NiOx by atmospheric pressure spatial atomic layer deposition (s-ALD) system, a more rapid method than conventional ALD [65,66], in p-i-n structured PSCs has been demonstrated. Employing high-quality and high-uniformity NiOx HTLs in PSCs resulted in PCEs over 17% with negligible hysteresis and fill factors over 80% [22,67]. Perovskite films with improved efficient collection of charge carriers and intrinsic electronic quality were enabled from the high uniformity of NiOx, resulting in PSC devices with reduced interfacial trapping and improved open-circuit voltage (VOC). NiO has also   Application of NiOx by atmospheric pressure spatial atomic layer deposition (s-ALD) system, a more rapid method than conventional ALD [65,66], in p-i-n structured PSCs has been demonstrated. Employing high-quality and high-uniformity NiOx HTLs in PSCs resulted in PCEs over 17% with negligible hysteresis and fill factors over 80% [22,67]. Perovskite films with improved efficient collection of charge carriers and intrinsic electronic quality were enabled from the high uniformity of NiOx, resulting in PSC devices with reduced interfacial trapping and improved open-circuit voltage (VOC). NiO has also been applied to p-i-n PSCs using plasma-enhanced ALD (PEALD), resulting in PSCs over 17% [24]. For p-i-n structured PSCs, atomic layer deposition (ALD) of NiO employed in PSCs resulted in efficiencies of over 18% [23]. The ALD NiO-based PSCs maintained over 99% of their initial PCE at room temperature conditions and 87% at 85 °C under 1 SUN, MPPT, as shown in Figure 4. Researchers have found various methods to dope NiOx to improve conductivity, and thus, the PSC device performance. Copper-doped NiOx (Cu:NiOx) resulted in efficiencies over 17.8% using a low-temperature combustion process, outperforming the conventional sol-gel-derived high-temperature Cu:NiOx PCE of 15.5% [25]. Despite the reduced process temperature Cu:NiOx prepared by this combustion process has a tendency to be better than the conventional high-temperature sol-gel process in terms of optical transparency, Researchers have found various methods to dope NiO x to improve conductivity, and thus, the PSC device performance. Copper-doped NiO x (Cu:NiO x ) resulted in efficiencies over 17.8% using a low-temperature combustion process, outperforming the conventional sol-gel-derived high-temperature Cu:NiO x PCE of 15.5% [25]. Despite the reduced process temperature Cu:NiO x prepared by this combustion process has a tendency to be better than the conventional high-temperature sol-gel process in terms of optical transparency, crystallinity, and electrical conductivity. Yue et al. further improved Cu:NiO x -based PSCs by doping the methylammonium lead halide perovskite with chlorine to improve the opencircuit voltage, modifying the aluminum cathode with zirconium acetylacetonate (Zracac), and employing fluorine-doped tin oxide (FTO), which resulted in a PCE of 20.5% [26]. Doping of NiO x with lithium (Li) and magnesium (Mg) in PSC has also been demonstrated, Nanomaterials 2022, 12, 112 9 of 28 resulting in PCEs of 18% [28] and 18.5% [30], respectively. Li-doped NiO surface by a hotcasting method enabled highly crystalline MAPbI 3 , resulting in hysteresis-free, efficient, and photostable PSCs. Limitations of poor fill factor and short-circuit current density in sputtered NiO x -based PSCs can be overcome through introducing Mg at a low oxygen partial pressure deposition condition. Doping NiO x with Li and Mg have also demonstrated devices with PCEs of above 20.7% with suitable stability for 500 h in a light soaking and thermal aging test [21], and 19.2% with retaining >80% of the initial PCE after light soaking for 1000 h or thermal exposure at 85 • C for 500 h [27]. Doping NiO x with cesium (Cs) exhibits higher conductivity and higher working function, resulting in improved PCE from 16.0% to 19.4% when applied in PSCs [29]. Doping with cobalt (Co) improved PSC devices from efficiencies of 16.0% to 17.8% due to less charge accumulation and open-circuit voltage loss from the improved hole mobility and reduction in interfacial resistance [31]. Thus, NiO x can be doped by various elements, such as Cu, Li, Mg, Cs, and Co, to improve the conductivity and enhance the efficiency of the PSC by reducing interfacial resistance at the HTL/perovskite interface.

Copper Thiocyanate
Copper thiocyanate (CuSCN) has a wide bandgap of 3.9 eV and appropriate valence band energy level of −5.3 eV with carrier mobility of 10 −2 -10 −1 cm 2 /Vs and superior thermal stability compared to spiro-OMeTAD, making it a suitable HTL candidate for PSCs [68]. Furthermore, it is solution processable with a low cost, showing potential in commercialization. CuSCN has been applied to PSCs in mostly n-i-p configurations. Madhavan [33]. CuSCN-based PSCs retained 60% of the initial PCE in air at 125 • C with 40% relative humidity for 2 h, while spiro-OMeTAD-based PSCs retained only 25% of their initial PCE under the same conditions.
Highly conformal CuSCN layers were formed through a fast solvent removal method, facilitating rapid carrier extraction and collection, resulting in a PCE of 20.4% [34]. Applying a reduced graphene oxide (RGO) layer before the top Au contact further enhances the stability by reducing potential-induced degradation from the reaction of Au and SCN − anions at the CuSCN/Au contact. These PSCs showed excellent thermal stability under long-term heating. Over 95% of the initial PCE is maintained under 1 SUN, 60 • C, in nitrogen (N 2 ) atmosphere with MPPT conditions after 1000 h surpassing the light stability of devices based on spiro-OMeTAD, as shown in Figure 5. Light and heat stability of CuSCN-based PSCs can be greatly enhanced by the insertion of an RGO layer in between CuSCN and Au to block anion diffusion. Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 28

Copper Iodide
Copper iodide (CuI) is a p-type semiconductor with a valence band energy level of −5.2 eV, a large bandgap of 3.1 eV, and hole mobility of 0.5-2 cm 2 /Vs [69]. Due to its hydrophobicity, CuI shows suitable ambient stability compared to PEDOT:PSS. CuI has already been widely used in OPV and DSSCs as an HTL and is a promising alternative in terms of low-cost and large-scale industrial commercialization [70]. CuI has been applied to PSCs in both p-i-n and n-i-p configurations, with PCEs of 7.5% [35] and 16.8% [36], respectively. CuI in the n-i-p structured device was successfully applied in a planar structured device and displayed significantly reduced hysteresis compared to the conventional devices based on spiro-OMeTAD. CuI in a p-i-n structured device maintained 93% of its initial PCE after storage in 25% humidity at room temperature without illumination for 300 h, showing better air stability than the reference PSC based on PEDOT:PSS, as shown in Figure 6

Copper Iodide
Copper iodide (CuI) is a p-type semiconductor with a valence band energy level of −5.2 eV, a large bandgap of 3.1 eV, and hole mobility of 0.5-2 cm 2 /Vs [69]. Due to its hydrophobicity, CuI shows suitable ambient stability compared to PEDOT:PSS. CuI has already been widely used in OPV and DSSCs as an HTL and is a promising alternative in terms of low-cost and large-scale industrial commercialization [70]. CuI has been applied to PSCs in both p-i-n and n-i-p configurations, with PCEs of 7.5% [35] and 16.8% [36], respectively. CuI in the n-i-p structured device was successfully applied in a planar structured device and displayed significantly reduced hysteresis compared to the conventional devices based on spiro-OMeTAD. CuI in a p-i-n structured device maintained 93% of its initial PCE after storage in 25% humidity at room temperature without illumination for 300 h, showing better air stability than the reference PSC based on PEDOT:PSS, as shown in Figure 6

Copper Oxide
Copper oxides, such as cuprous oxide (Cu2O) and cupric oxide (CuO), are p-type semiconductors composed of environmentally friendly and abundant elements with low cost and suitable heat and ambient stability [71]. CuO has a bandgap of 1.3 eV and a valence band energy level of approximately −5.4 eV, while Cu2O has a bandgap of 2.1 eV, valence band energy level of −5.3 to −5.4 eV, and high carrier mobility of ~100 cm 2 /Vs [72]. Solution-processed CuOx has been applied to p-i-n PSC devices, exhibiting high transparency in the visible region and a smooth surface, resulting in a PCE of 17.1% [38]. Approximately 90% of the initial PCE was maintained after storage in air without encapsulation for 300 h, showing enhanced air stability compared to the conventional PSC device based on PEDOT:PSS, as shown in Figure 7. Efficiencies were further improved to 17.4% by applying solution-processed CuOx, which exhibit high optical transmittance, high work function, and excellent hole-extracting ability [40]. Conventional PEDOT:PSSbased PSC devices resulted in 12.0% efficiency. The high work function of CuOx enables ohmic contact at the perovskite/CuOx interface, which reduces open-circuit voltage loss.

Copper Oxide
Copper oxides, such as cuprous oxide (Cu 2 O) and cupric oxide (CuO), are p-type semiconductors composed of environmentally friendly and abundant elements with low cost and suitable heat and ambient stability [71]. CuO has a bandgap of 1.3 eV and a valence band energy level of approximately −5.4 eV, while Cu 2 O has a bandgap of 2.1 eV, valence band energy level of −5.3 to −5.4 eV, and high carrier mobility of~100 cm 2 /Vs [72]. Solution-processed CuO x has been applied to p-i-n PSC devices, exhibiting high transparency in the visible region and a smooth surface, resulting in a PCE of 17.1% [38]. Approximately 90% of the initial PCE was maintained after storage in air without encapsulation for 300 h, showing enhanced air stability compared to the conventional PSC device based on PEDOT:PSS, as shown in Figure 7. Efficiencies were further improved to 17.4% by applying solution-processed CuO x , which exhibit high optical transmittance, high work function, and excellent hole-extracting ability [40]. Conventional PEDOT:PSS-based PSC devices resulted in 12.0% efficiency. The high work function of CuO x enables ohmic contact at the perovskite/CuO x interface, which reduces open-circuit voltage loss. Further improvements in CuOx-based PSCs were made by Rao et al. by Cl doping of the perovskite layer based on a modified one-step fast deposition-crystallization method leading to a PCE of 19.0% [39]. Cl-doping MAPbI3 perovskite films remarkably improves the perovskite hole mobility and film morphology, greatly increasing the device recombination resistance and reducing the intrinsic defects. Quantum dot (QD) Cu2O dispersed in a nonpolar solvent has been spin-coated on top of the perovskite layer in a mesoporous n-i-p structure, resulting in a PCE of 18.9% [41]. Surface modification of Cu2O allows direct deposition on the perovskite film without decomposing the perovskite, resulting in a significantly higher PCE compared to the unmodified Cu2O, which resulted in a PCE of 11.9%. The dopant-free method and hydrophobic surface of Cu2O enable excellent long-term stability maintaining over 90% of the initial PCE for over 1 month when stored in air without encapsulation with a relative humidity of 30%, as shown in Figure 8. Kim et al. reported a one-step deposition of Cu2O-CuSCN to produce a nanocomposite HTL composed of Cu2O nanoparticles (20 nm in size) dispersed in a CuSCN solution with diethyl sulfide [42]. High mobility of Cu2O placed at the perovskite/CuSCN interface improved the hole extraction rate and reduced interfacial reaction, improving the PSC efficiencies from 17.7 to 19.2%, and encapsulated devices sustained its PCE over 90% under severe conditions of 85% relative humidity and 85 °C for 720 h, as shown in Figure 9. Further improvements in CuO x -based PSCs were made by Rao et al. by Cl doping of the perovskite layer based on a modified one-step fast deposition-crystallization method leading to a PCE of 19.0% [39]. Cl-doping MAPbI 3 perovskite films remarkably improves the perovskite hole mobility and film morphology, greatly increasing the device recombination resistance and reducing the intrinsic defects. Quantum dot (QD) Cu 2 O dispersed in a nonpolar solvent has been spin-coated on top of the perovskite layer in a mesoporous n-i-p structure, resulting in a PCE of 18.9% [41]. Surface modification of Cu 2 O allows direct deposition on the perovskite film without decomposing the perovskite, resulting in a significantly higher PCE compared to the unmodified Cu 2 O, which resulted in a PCE of 11.9%. The dopant-free method and hydrophobic surface of Cu 2 O enable excellent long-term stability maintaining over 90% of the initial PCE for over 1 month when stored in air without encapsulation with a relative humidity of 30%, as shown in Figure 8 [42]. High mobility of Cu 2 O placed at the perovskite/CuSCN interface improved the hole extraction rate and reduced interfacial reaction, improving the PSC efficiencies from 17.7 to 19.2%, and encapsulated devices sustained its PCE over 90% under severe conditions of 85% relative humidity and 85 • C for 720 h, as shown in Figure 9.

Delafossites
Delafossite materials are based on the chemical formula of ABO2, where A is Cu, Pt, Pd, or Ag, and B is Al, Ga, Cr, In, Sc, Fe, Y, La, etc. Some common Cu-based delafossite materials are CuAlO2, CuCrO2, and CuGaO2. CuAlO2 has a bandgap of 3.75-3.86 eV, valence band of −5.0 to −5.3 eV, and hole mobility of 3.6 cm 2 /Vs [73,74]. p-Type CuAlO2 has been reported to exhibit decent thermal, chemical, and ambient stability and optical transparency and contains non-toxic and cheap, easily accessible elements. Inserting CuAlO2 deposited by direct current (DC) magnetron sputtering on top of ITO and below PEDOT:PSS in a p-i-n configuration resulted in a higher PCE of 14.5% compared to the PCE of the reference device (11.1%) [43]. By inserting 15 nm of CuAlO2, the stability of the device improved by maintaining 80% of its initial PCE after storage in ambient conditions for 240 h, whereas the reference device only retained 35% of its initial PCE.
CuCrO2 has a bandgap of 2.9-3.1 eV while maintaining high transmittance in the wavelength region above 400 nm, valence band energy level of −5.3 eV, and carrier mobility of 0.1-1 cm 2 /Vs with suitable light stability [75]. CuCrO2 spin-coated on top of the perovskite layer in an n-i-p structure resulted in a PCE of 16.7%, and retained around 88% of its initial PCE after 500 h under 1 SUN, MPPT in a nitrogen atmosphere at room temperature, as shown in Figure 10 [12]. In a p-i-n structure, applying CuCrO2 nanocrystals as the HTL resulted in an efficiency of 19% [44] and retained ~95% of its initial PCE after continuous 1 SUN illumination in argon atmosphere for 1,000 h, as shown in Figure 11. Here, CuCrO2 nanocrystals function as an HTL as well as a UV-blocking underlayer to improve photostability. CuCrO2 has also been doped with Mg, with improved conductivity from 1 to 220 S cm −1 , resulting in a PCE of 13.1% [45]. The PCE was further improved by Zhang et al. to 14.1% [46].

Delafossites
Delafossite materials are based on the chemical formula of ABO 2 , where A is Cu, Pt, Pd, or Ag, and B is Al, Ga, Cr, In, Sc, Fe, Y, La, etc. Some common Cu-based delafossite materials are CuAlO 2 , CuCrO 2 , and CuGaO 2 . CuAlO 2 has a bandgap of 3.75-3.86 eV, valence band of −5.0 to −5.3 eV, and hole mobility of 3.6 cm 2 /Vs [73,74]. p-Type CuAlO 2 has been reported to exhibit decent thermal, chemical, and ambient stability and optical transparency and contains non-toxic and cheap, easily accessible elements. Inserting CuAlO 2 deposited by direct current (DC) magnetron sputtering on top of ITO and below PEDOT:PSS in a p-i-n configuration resulted in a higher PCE of 14.5% compared to the PCE of the reference device (11.1%) [43]. By inserting 15 nm of CuAlO 2 , the stability of the device improved by maintaining 80% of its initial PCE after storage in ambient conditions for 240 h, whereas the reference device only retained 35% of its initial PCE. CuCrO 2 has a bandgap of 2.9-3.1 eV while maintaining high transmittance in the wavelength region above 400 nm, valence band energy level of −5.3 eV, and carrier mobility of 0.1-1 cm 2 /Vs with suitable light stability [75]. CuCrO 2 spin-coated on top of the perovskite layer in an n-i-p structure resulted in a PCE of 16.7%, and retained around 88% of its initial PCE after 500 h under 1 SUN, MPPT in a nitrogen atmosphere at room temperature, as shown in Figure 10 [12]. In a p-i-n structure, applying CuCrO 2 nanocrystals as the HTL resulted in an efficiency of 19% [44] and retained~95% of its initial PCE after continuous 1 SUN illumination in argon atmosphere for 1000 h, as shown in Figure 11. Here, CuCrO 2 nanocrystals function as an HTL as well as a UV-blocking underlayer to improve photostability. CuCrO 2 has also been doped with Mg, with improved conductivity from 1 to 220 S cm −1 , resulting in a PCE of 13.1% [45]. The PCE was further improved by Zhang et al. to 14.1% [46].  CuGaO2 has a bandgap of 3.6 eV, a valence band energy level of ~−5.3 eV, and hole mobility of 10 −2 -10 −1 cm 2 /Vs [76][77][78]. CuGaO2 has suitable heat and ambient stability compared to spiro-OMeTAD. CuGaO2 spin-coated on top of the perovskite layer in an ni-p configuration resulted in a PCE of 18.5% and retained over 90% of its initial PCE after storage in ambient air at 25 °C and 30-55% relative humidity for 30 days without encapsulation, which is superior to the spiro-OMeTAD-based PSCs, as shown in Figure  12 [47]. A mesoporous CuGaO2 coated on top of NiOx on FTO resulted in a PCE of 20%, which is superior to that of the planar cell (16.7%) [48], and maintained over 80% of its original PCE after 1000 h in a nitrogen atmosphere at 85 °C of unencapsulated devices, as shown in Figure 13. CuGaO 2 has a bandgap of 3.6 eV, a valence band energy level of~−5.3 eV, and hole mobility of 10 −2 -10 −1 cm 2 /Vs [76][77][78]. CuGaO 2 has suitable heat and ambient stability compared to spiro-OMeTAD. CuGaO 2 spin-coated on top of the perovskite layer in an n-i-p configuration resulted in a PCE of 18.5% and retained over 90% of its initial PCE after storage in ambient air at 25 • C and 30-55% relative humidity for 30 days without encapsulation, which is superior to the spiro-OMeTAD-based PSCs, as shown in Figure 12 [47]. A mesoporous CuGaO 2 coated on top of NiO x on FTO resulted in a PCE of 20%, which is superior to that of the planar cell (16.7%) [48], and maintained over 80% of its original PCE after 1000 h in a nitrogen atmosphere at 85 • C of unencapsulated devices, as shown in Figure 13.
i-p configuration resulted in a PCE of 18.5% and retained over 90% of its initial PCE after storage in ambient air at 25 °C and 30-55% relative humidity for 30 days without encapsulation, which is superior to the spiro-OMeTAD-based PSCs, as shown in Figure  12 [47]. A mesoporous CuGaO2 coated on top of NiOx on FTO resulted in a PCE of 20%, which is superior to that of the planar cell (16.7%) [48], and maintained over 80% of its original PCE after 1000 h in a nitrogen atmosphere at 85 °C of unencapsulated devices, as shown in Figure 13. CuFeO 2 is also a cost-effective and highly light, moisture, and thermally stable material for an HTL candidate. PSCs with CuFeO 2 exhibit suitable thermal, moisture, and photostability compared to PSCs based on spiro-OMeTAD [49]. Unencapsulated devices with CuFeO 2 retained about 85% of their initial PCE under 1 SUN at MPPT for over 1000 h in nitrogen, whereas spiro-OMeTAD devices dropped to 10%, as shown in Figure 14. Thermal and humidity stability tests show that CuFeO 2 -based devices retained 80% of their initial PCE after exposure to 70 • C for 120 h and retained over 90% of their initial PCE after exposure to 80 ± 5% relative humidity for 300 h. Among the delafossite-based PSCs, devices with CuCrO 2 and CuGaO 2 reported high efficiencies above 19%. PSCs with CuCrO 2 show suitable light stability, while CuGaO 2 -based devices show suitable heat and ambient stability compared to spiro-OMeTAD-based devices.
(c) CuFeO2 is also a cost-effective and highly light, moisture, and thermally stable material for an HTL candidate. PSCs with CuFeO2 exhibit suitable thermal, moisture, and photostability compared to PSCs based on spiro-OMeTAD [49]. Unencapsulated devices with CuFeO2 retained about 85% of their initial PCE under 1 SUN at MPPT for over 1000 h in nitrogen, whereas spiro-OMeTAD devices dropped to 10%, as shown in Figure 14. Thermal and humidity stability tests show that CuFeO2-based devices retained 80% of their initial PCE after exposure to 70 °C for 120 h and retained over 90% of their initial PCE after exposure to 80% ± 5% relative humidity for 300 h. Among the delafossite-based PSCs, devices with CuCrO2 and CuGaO2 reported high efficiencies above 19%. PSCs with CuCrO2 show suitable light stability, while CuGaO2-based devices show suitable heat and ambient stability compared to spiro-OMeTAD-based devices.

Copper Sulfide
Copper sulfide (CuS) is a p-type semiconductor, which has also been used in the fields of gas sensors, catalysis, and nonlinear optical materials [79,80]. CuS has been investigated to replace PEDOT:PSS in OPV, exhibiting decent performance compared to devices based on PEDOT:PSS [81]. CuS nanoparticles were coated on top of ITO in a p-i-n configuration PSCs, which resulted in a PCE of 16.2% [50], and maintained over 90% of its initial PCE in air without encapsulation for 260 h, shown in Figure 15. CuS nanoparticles can modify the surface of ITO by tuning the surface work function, reducing the interfacial carrier injection barrier, and enabling the hole extraction efficiency between

Copper Sulfide
Copper sulfide (CuS) is a p-type semiconductor, which has also been used in the fields of gas sensors, catalysis, and nonlinear optical materials [79,80]. CuS has been investigated to replace PEDOT:PSS in OPV, exhibiting decent performance compared to devices based on PEDOT:PSS [81]. CuS nanoparticles were coated on top of ITO in a p-i-n configuration PSCs, which resulted in a PCE of 16.2% [50], and maintained over 90% of its initial PCE in air without encapsulation for 260 h, shown in Figure 15. CuS nanoparticles can modify the surface of ITO by tuning the surface work function, reducing the interfacial carrier injection barrier, and enabling the hole extraction efficiency between the ITO and perovskite layers, but not ruin the transmittance and surface roughness of ITO.
(e) Figure 14. CuFeO2-incorporated PSC: (a) Energy band diagram with respect to the vacuum level. (b) Illuminated J-V scans of the champion cells with different HTLs. (c) Normalized PCE evolution of unencapsulated devices under continuous 1 SUN illumination, MPPT in a nitrogen atmosphere. Normalized PCE decay over time in various (d) humidity and (e) temperature conditions. Reproduced from the work of [49], with permission from the American Chemical Society, 2019.

Copper Sulfide
Copper sulfide (CuS) is a p-type semiconductor, which has also been used in fields of gas sensors, catalysis, and nonlinear optical materials [79,80]. CuS has b investigated to replace PEDOT:PSS in OPV, exhibiting decent performance compare devices based on PEDOT:PSS [81]. CuS nanoparticles were coated on top of ITO in a p configuration PSCs, which resulted in a PCE of 16.2% [50], and maintained over 90% its initial PCE in air without encapsulation for 260 h, shown in Figure 15. nanoparticles can modify the surface of ITO by tuning the surface work function, redu the interfacial carrier injection barrier, and enabling the hole extraction efficiency betw the ITO and perovskite layers, but not ruin the transmittance and surface roughnes ITO.

Cobalt Oxide
Cobalt oxide (CoOx) has a favorable valence band energy level of −5.3 eV. Co3O4 applied by screen printing to an n-i-p configuration of a ZrO2 scaffold resulted in a PCE of 13.3% for carbon-based PSCs [52]. CoOx spin-coated on top of ITO in a p-i-n configuration resulted in a PCE of 14.5% [51]. Shalan et al. reported that according to photoluminescence decays of perovskite deposited on various HTLs, CoOx had a faster hole-extracting time of 2.8 ns compared to PEDOT:PSS (17.5 ns) and NiOx (22.8 ns). CoOx-based PSC retained 90% of its initial PCE after storage in a nitrogen atmosphere for 1,000 h. Lithium cobalt oxide (LiCoO2) prepared by radio frequency (RF) magnetron sputtering in a p-i-n structured device resulted in a PCE of 19.1%, with high efficiency stable up to 90 °C, and 60% of the initial PCE was retained after continuous thermal stress at 100 °C for 5 days in an inert atmosphere, showing higher stability than the PEDOT:PSS-based device (Figure

Cobalt Oxide
Cobalt oxide (CoO x ) has a favorable valence band energy level of −5.3 eV. Co 3 O 4 applied by screen printing to an n-i-p configuration of a ZrO 2 scaffold resulted in a PCE of 13.3% for carbon-based PSCs [52]. CoO x spin-coated on top of ITO in a p-i-n configuration resulted in a PCE of 14.5% [51]. Shalan et al. reported that according to photoluminescence decays of perovskite deposited on various HTLs, CoO x had a faster hole-extracting time of 2.8 ns compared to PEDOT:PSS (17.5 ns) and NiO x (22.8 ns). CoO x -based PSC retained 90% of its initial PCE after storage in a nitrogen atmosphere for 1000 h. Lithium cobalt oxide (LiCoO 2 ) prepared by radio frequency (RF) magnetron sputtering in a p-i-n structured device resulted in a PCE of 19.1%, with high efficiency stable up to 90 • C, and 60% of the initial PCE was retained after continuous thermal stress at 100 • C for 5 days in an inert atmosphere, showing higher stability than the PEDOT:PSS-based device ( Figure 16) [53]. UV-ozone-treated LiCoO 2 exhibits a super-hydrophilic surface that can be wetted easily by the perovskite precursor solution and made wetting of a large-area substrate of 10 cm × 10 cm possible.

Cobalt Oxide
Cobalt oxide (CoOx) has a favorable valence band energy level of −5.3 eV. Co3O4 applied by screen printing to an n-i-p configuration of a ZrO2 scaffold resulted in a PCE of 13.3% for carbon-based PSCs [52]. CoOx spin-coated on top of ITO in a p-i-n configuration resulted in a PCE of 14.5% [51]. Shalan et al. reported that according to photoluminescence decays of perovskite deposited on various HTLs, CoOx had a faster hole-extracting time of 2.8 ns compared to PEDOT:PSS (17.5 ns) and NiOx (22.8 ns). CoOx-based PSC retained 90% of its initial PCE after storage in a nitrogen atmosphere for 1,000 h. Lithium cobalt oxide (LiCoO2) prepared by radio frequency (RF) magnetron sputtering in a p-i-n structured device resulted in a PCE of 19.1%, with high efficiency stable up to 90 °C, and 60% of the initial PCE was retained after continuous thermal stress at 100 °C for 5 days in an inert atmosphere, showing higher stability than the PEDOT:PSS-based device ( Figure  16) [53]. UV-ozone-treated LiCoO2 exhibits a super-hydrophilic surface that can be wetted easily by the perovskite precursor solution and made wetting of a large-area substrate of 10 cm × 10 cm possible.

Chromium Oxide
Chromium oxide (CrOx) has also been investigated to replace organic HTLs in PSCs. Cu doping of CrOx can suppress the surface hydroxylation and hexavalent chromium ions, which are harmful to the interface stability of PSCs. Cu-doped CrOx in p-i-n PSC devices resulted in a PCE of 17.7%, and maintained over 70% of its original PCE after 190 h in 30% humidity 20 °C without encapsulation, as shown in Figure 17, whereas undoped CrOx-based PSCs resulted in a PCE of 14.8% and maintained less than 10% of its initial PCE [54]. PSCs with Cu-doped CrOx shows superior ambient stability than PSCs with undoped CrOx.

Chromium Oxide
Chromium oxide (CrO x ) has also been investigated to replace organic HTLs in PSCs. Cu doping of CrO x can suppress the surface hydroxylation and hexavalent chromium ions, which are harmful to the interface stability of PSCs. Cu-doped CrO x in p-i-n PSC devices resulted in a PCE of 17.7%, and maintained over 70% of its original PCE after 190 h in 30% humidity 20 • C without encapsulation, as shown in Figure 17, whereas undoped CrO x -based PSCs resulted in a PCE of 14.8% and maintained less than 10% of its initial PCE [54]. PSCs with Cu-doped CrO x shows superior ambient stability than PSCs with undoped CrO x .
Cu doping of CrOx can suppress the surface hydroxylation and hexavalent chromium ions, which are harmful to the interface stability of PSCs. Cu-doped CrOx in p-i-n PSC devices resulted in a PCE of 17.7%, and maintained over 70% of its original PCE after 190 h in 30% humidity 20 °C without encapsulation, as shown in Figure 17, whereas undoped CrOx-based PSCs resulted in a PCE of 14.8% and maintained less than 10% of its initial PCE [54]. PSCs with Cu-doped CrOx shows superior ambient stability than PSCs with undoped CrOx.

Molybdenum Oxide
Molybdenum oxide (MoO3) is an n-type semiconductor with a deep conduction band energy level, making it an appropriate HTL. Due to its suitable energy alignment properties, MoO3 has already been used in OPV [82]. Thermally evaporated MoOx on ITO in a p-i-n configuration resulted in a PCE of 13.1% [55]. UV-ozone treatment of MoOx was required to increase the wettability of the perovskite formation process. Titanium-doped MoO2 nanoparticles by a scalable solvothermal cracking process applied to an n-i-p PSC configuration resulted in a PCE of 15.8% [56]. Titanium-doping in MoO2 nanoparticles produces stronger Mo-O bonding and thus, enhances the stability against humidity. Xie et al. reported that reduced graphene oxide (RGO) doping is an effective method to make MoOx a promising HTL [57]. Conductive MoOx:RGO can facilitate perovskite crystallization and reduce the VOC loss, resulting in a PCE of 18.2% and VOC of 1.12 V.

Vanadium Oxide
Vanadium oxide (VO2) is also an n-type semiconductor with a deep conduction band energy level, making it an appropriate HTL [82,83]. VOx on top of the perovskite layer using a ZrO2 scaffold resulted in a PCE of 15.8% [58]. VOx was applied by post-treatment of the perovskite/carbon interface to facilitate the charge transfer from the high work function of VOx while not sacrificing the conductivity of carbon. A low-temperature solution-processed Cs-doped VOx was applied on top of ITO in a p-i-n structure resulted in a PCE of 14.5% [59], and maintained 94% of its initial PCE value after 720 h in the air (50-70% humidity) without encapsulation, showing suitable ambient stability, as shown in Figure 18. Introducing Cs to VOx improved the electrical conductivity and can change

Molybdenum Oxide
Molybdenum oxide (MoO 3 ) is an n-type semiconductor with a deep conduction band energy level, making it an appropriate HTL. Due to its suitable energy alignment properties, MoO 3 has already been used in OPV [82]. Thermally evaporated MoO x on ITO in a p-i-n configuration resulted in a PCE of 13.1% [55]. UV-ozone treatment of MoO x was required to increase the wettability of the perovskite formation process. Titanium-doped MoO 2 nanoparticles by a scalable solvothermal cracking process applied to an n-i-p PSC configuration resulted in a PCE of 15.8% [56]. Titanium-doping in MoO 2 nanoparticles produces stronger Mo-O bonding and thus, enhances the stability against humidity. Xie et al. reported that reduced graphene oxide (RGO) doping is an effective method to make MoO x a promising HTL [57]. Conductive MoO x :RGO can facilitate perovskite crystallization and reduce the V OC loss, resulting in a PCE of 18.2% and V OC of 1.12 V.

Vanadium Oxide
Vanadium oxide (VO 2 ) is also an n-type semiconductor with a deep conduction band energy level, making it an appropriate HTL [82,83]. VO x on top of the perovskite layer using a ZrO 2 scaffold resulted in a PCE of 15.8% [58]. VO x was applied by post-treatment of the perovskite/carbon interface to facilitate the charge transfer from the high work function of VO x while not sacrificing the conductivity of carbon. A low-temperature solution-processed Cs-doped VO x was applied on top of ITO in a p-i-n structure resulted in a PCE of 14.5% [59], and maintained 94% of its initial PCE value after 720 h in the air (50-70% humidity) without encapsulation, showing suitable ambient stability, as shown in Figure 18. Introducing Cs to VO x improved the electrical conductivity and can change the phase separation pattern and microstructural film morphology. The enlarged surface roughness resulted in enhanced interfacial adhesion between the HTL and perovskite layer.

Conclusions
In summary, the recent progress of inorganic HTL-based PSCs and the roles of the inorganic HTLs on device performance and stability against heat, humidity, bias, and light are discussed. The efficiencies of inorganic HTL-based PSCs reported so far are over 20% for Cu:NiOx, Li,Mg:NiOx, CuSCN, CuI-CuSCN, and CuGaO2, which is lower but still approaching the efficiencies for organic HTL-based PSCs. Superior device stability of inorganic HTL-based PSCs to organic HTL-based PSCs has been reported, showing the

Conclusions
In summary, the recent progress of inorganic HTL-based PSCs and the roles of the inorganic HTLs on device performance and stability against heat, humidity, bias, and light are discussed. The efficiencies of inorganic HTL-based PSCs reported so far are over 20% for Cu:NiO x , Li,Mg:NiO x , CuSCN, CuI-CuSCN, and CuGaO 2 , which is lower but still approaching the efficiencies for organic HTL-based PSCs. Superior device stability of inorganic HTL-based PSCs to organic HTL-based PSCs has been reported, showing the potential of inorganic HTLs to replace organic HTLs in PSC devices. Further investigation on increasing the PCEs of inorganic HTL-based PSCs and a better understanding of the degradation and working mechanisms are still required.
The general criteria for selecting potential HTL candidates are also discussed. The valence band energy levels should be close to that of the perovskite layer to facilitate efficient carrier transport and appropriate conduction band energy levels to impede recombination at the HTL/perovskite interface. The carrier mobility should be high to reduce resistance and loss during transport, while the transparency should be high enough to reduce input solar radiation loss. In a p-i-n structure, the nucleation and wettability of the perovskite solution on the HTL surface become important. In an n-i-p structure, the stability of the HTL becomes important because of its contact with humidity and oxygen.
Although this review focused on single-junction unit-cell perovskite solar cells, largearea coating methods and tandem configurations, which consist of wide-bandgap perovskite solar cells on top of lower bandgap materials, such as silicon, Cu(In,Ga)Se 2 , and tin-related materials [15,[84][85][86][87][88], need to be considered for commercialization [89]. Thus, inorganic HTL incorporated into tandem perovskite solar cells and coating methods for large-area devices are future directions to be taken. There may be temperature or fabrication limitations of the inorganic HTL in these tandem configurations, depending on the bottom cell. Especially, flexible tandem solar cells will have a limit on the processing temperatures of the layers. Obtaining highly uniform pinhole-free films of the inorganic HTL over a large area will also be important to consider for commercialization.